Last year, uptake of home DNA tests sailed into the stratosphere, giving millions of people around the world a whole new level of insight into the ways their genes work, and helping predict their genetic risk for disease.

But knowledge is a dangerous thing. New research suggests this kind of genetic knowledge isn’t just informative, but can also be transformative – with DNA results conferring a kind of genetic placebo effect that actually impacts how people’s bodies function.

“Receiving genetic information doesn’t just make you more informed,” explains psychologist and senior researcher of the new study, Alia Crum from Stanford University.

“What this study shows is that it can also have a physiological impact on your body in a way that actually changes your overall risk profile.”

In an experiment, the researchers took DNA samples from over 200 participants. Then, the group was divided into two smaller groups, each with more than 100 members.

One of these groups performed an exercise test on a treadmill, while the other group ate a small meal.

A week later, the participants returned for a repeat of their exercise (or meal-eating) experiment, but with a crucial difference. This time, before doing the second test, they were given information on the results of their genetic test – with a catch.

Specifically, instead of telling the participants their real genetic results, they were told random results. The exercise group was informed they either possessed a gene variant associated with poorer aerobic exercise capacity, or a gene that gave them extra endurance.

Similarly, one half of the eating group was randomly informed they could be genetically predisposed to obesity due to a gene variant that made it harder to feel full after eating; the others being told their genes promoted feelings of satiety, helping protect them from obesity.

After being given these deceptive, random results – that bore no relation to the people’s actual genotype – the mere conveyance of this false information affected how they performed in their next tests.

Those on the running machine who had the impression their aerobic capacity was impaired, struggled to run – and quit sooner than on their first test. Those who believed they had the protective gene variant, on the other hand, performed about the same.

In the eating test, those who were told they could be predisposed to obesity did about the same as before, rating their levels of fullness similarly, and producing similar levels of a fullness hormone in their blood.

But the participants who were told they were protected from obesity by virtue of their satiety-promoting gene actually produced about 250 percent more of the fullness hormone in their blood than in the blind test a week before.

“It was really a much stronger and faster physiological satiety signal, and this mapped on to how much more full participants said that they felt,” first author of the study, Bradley Turnwald, explains.

So what does this tell us? The researchers are eager to emphasise it doesn’t mean getting your genome sequenced is a bad idea, but they say people need to be aware that the genetic information itself can have a measurable effect on our physiology, even if we’re not genetically predisposed to certain things.

“The take-home message here is that the mindset that you put people in when you deliver genetic risk information is not irrelevant,” Crum says.

“The mindset of being genetically at risk or protected can alter how we feel, what we do and – as this study shows – how our bodies respond.”

But given the ongoing explosion in consumer DNA sequencing, this is one important side effect people need to be aware of – after all, nobody reads their genome results in a vacuum.

“Although much remains to be explored, the present research represents a major advance in our understanding of the impact of genetic risk disclosure and suggests that learning of one’s genetic risk of obesity may in fact exacerbate one’s risk,” the authors write.

“The results herein underscore the critical need to accompany biological advances in genetics with an equally sophisticated understanding of the impact of receiving genetic risk information on patient health outcomes.”